The C programming cos function is defined in the math.h header file and is used to calculate the cosine of a given value x, in radians.

The cos function takes a single argument of type double, which represents the value whose cosine is to be calculated. The function returns a double value, which represents the calculated cosine.

Here's an example usage of the cos function to calculate the cosine of a given input:

#include <stdio.h>
#include <math.h>

int main() {
    double x = 0.5;
    double cosine = cos(x);
    
    printf("The cosine of %f is %f\n", x, cosine);
    
    return 0;
}

In this example, the cos function is used to calculate the cosine of 0.5, which is stored in the x variable. The calculated cosine is then stored in the cosine variable, and printed to the console using the printf function.

Note that the cos function assumes that the input x is in radians. If the input is in degrees, it must be converted to radians first using the deg2rad function from the math.h header file. For example, to calculate the cosine of 30 degrees, you can use cos(deg2rad(30)).